The present invention relates to a cold isopressing method in which a green material is compacted within an isopressing mold. More particularly, the present invention relates to such a method in which two or more layers of green ceramic material are laminated within the isopressing mold and one of the layers is a tape-cast film. Even more particularly, the present invention relates to such a method in which the laminated layers are used to form a ceramic membrane element capable of selectively transporting oxygen or hydrogen ions.
Cold isopressing is a well-known technique that is used to form filters, structural elements and membranes. In isostatic pressing, a granular form of the material to be compacted is placed within an elastic isopressing mold. The granular material can be a ceramic or metallic powder or in case of ceramics can be a mixture of powder, binder and plasticizing agents. The isopressing mold is then positioned within a pressure vessel and slowly subjected to a hydrostatic pressure with cold or warm water to compact the granular material into a green form which subsequently, as appropriate, can be fired and sintered. An example of such a process that is applied to the formation of Tungsten rods is disclosed in U.S. Pat. No. 5,631,029. In this patent, fine Tungsten powder is isostatically pressed into a tungsten ingot.
Various green ceramics have been manufactured by isostatically pressing green ceramic materials. The isopressing molds can be cylindrical, as has been described above with reference to Tungsten ingots, or can be flat to produce plate-like articles. An important application for ceramic materials is to produce ceramic membrane elements to separate oxygen or hydrogen from feed streams. Such ceramic materials, while impermeable to the oxygen or hydrogen, conduct ions of oxygen or hydrogen to effect the separation. In practice, the ceramic is subjected to a high temperature and the oxygen or hydrogen is ionized at one surface of the membrane. The ions travel through the membrane and recombine at the other side thereof to emit electrons. The electrons are conducted through the ceramic material itself or through a separate electrical pathway for ionization purposes.
For example, a class of such materials, known as mixed conductors conduct both oxygen ions and electrons. These materials are well suited for oxygen separation since they can be operated in a pressure driven mode, that is a difference in oxygen activity on the two sides of the ceramic drives the oxygen transport. Perovskites such as La1xe2x88x92xSrxCoO3xe2x88x92y, LaxSr1xe2x88x92xFeO3xe2x88x92y, LaxSr1xe2x88x92xFe1xe2x88x92yCoyO3xe2x88x92z are examples of mixed conductors. At elevated temperatures, these materials contain mobile oxygen-ion vacancies [VO . . . ] that provide conduction sites for transport of oxygen ions through the material. Oxygen ions are transported selectively, and can thus act as a membrane with an infinite selectivity for oxygen. The oxygen transport involves the following chemical reactions:
1/2O2+2exe2x88x92 xe2x88x92⇄Oxe2x88x92 xe2x88x92Surface reaction
Oxe2x88x92 xe2x88x92+[VO . . . ]⇄nil Reaction within the electrolyte
The oxygen ions annihilate the highly mobile oxygen ion vacancies in the electrolyte. Electrons must be supplied (and removed at the other end of the membrane) for this reaction to proceed.
An oxygen partial pressure differential across the membrane gives rise to an electromotive force (emf) termed as Nernst potential, and is given by the following equation:
V=(RT/zF)ln(Po2,2/Po2, 1)
where,
R=the gas constant (8.314 J/gmole-K)
T=temperature (K)
F=Faraday""s constant (96488 Coulomb/gmole)
Po2, 1 and Po2, 2=partial pressure of oxygen on the opposite sides of the membrane
z=the number of electrons given up by one oxygen molecule, i.e. 4
The Nernst potential is developed internally, and it drives the flux of oxygen vacancies against the ionic resistance of the electrolyte. Thin films are therefore highly desirable because the ideal oxygen flux is inversely proportional to the thickness of the membrane. Thus, thinner films can lead to higher oxygen fluxes, reduced area, lower operating temperatures and smaller O2 pressure differentials across the electrolyte.
The thin films of ceramic are, however, fragile and must be supported. Therefore, efforts have been aimed at development of the thin film technology involving the deposition of a dense oxygen transport membrane film on a suitable porous substrate.
Solid state gas separation membranes, formed by depositing a dense mixed conducting oxide layer onto a porous mixed conducting support are disclosed in Yasutake Teraoka et al. xe2x80x9cDevelopment of Oxygen Semipermeable Membrane Using Mixed Conductive Perovskite-Type Oxidesxe2x80x9d Jour. Ceram. Soc. Japan. International Ed, Vol. 97, No. 4, pp 458-462, 1989 and Yasutake Teraoka et al. xe2x80x9cPreparation of Dense Film of Perovskite-Type Oxide on Porous Substratexe2x80x9d, Jour. Ceram. Soc. Japan, International Ed. Vol. 97, No. 5, pp 523-529, 1989. The relatively thick porous mixed conducting supports disclosed in these references provide mechanical stability for the thin, relatively fragile, dense mixed conducting layers. In these references, thin films composed of La0.6Sr0.4CoO3 were deposited onto porous supports of the same material by rf sputtering and liquid suspension spray deposition. The films produced by sputtering proved to be cracked and porous. Thin films (less than 15 xcexcm in thick thickness) made by liquid suspension spraying followed by sintering at 1400xc2x0 C. were dense and crack-free. Pal et al. xe2x80x9cElectrochemical Vapor Deposition of Yttria-Stabilized Zirconia Filmsxe2x80x9d from the Proceedings of the First International Symposium on Solid Oxide Fuel Cells, Vol. 89-11, pp 41-56, 1989 discloses an EVD process in which yttria-stabilized zirconia electrolyte films are deposited onto a porous substrate. EVD is a modification of the conventional chemical vapor deposition process which utilizes a chemical potential gradient to grow thin, gas impervious layers of either electronically, or ionically conducting metal oxides on porous substrates. The process involves contacting a mixture of metal halides on one side of a porous substrate and a mixture of hydrogen and water on the opposite side. The reactants diffuse into the substrate pores and react to form the multicomponent metal oxide that is deposited on the pore wall. Continued deposition, however, causes pore narrowing until eventually the pores are plugged with the multicomponent metal oxide.
U.S. Pat. No. 5,240,480 discloses an organometallic chemical deposition (OMCVD) method to prepare thin films of muticomponent metallic oxides for use as inorganic membranes. The inorganic membranes are formed by reacting organometallic complexes corresponding to each of the respective metals and an oxidizing agent under conditions sufficient to deposit a thin membrane onto the porous substrate.
Both EVD and OMCVD process involve expensive and complex equipment and often toxic and expensive precursor materials. Furthermore, it is difficult to control the stoichiometry of multicomponent metallic oxides (e.g. mixed conducting perovskites) deposited by such processes.
U.S. Pat. No. 5,494,700 discloses a precipitate- free aqueous solution containing a metal ion and a polymerizable organic solvent to fabricate dense crack- free thin films ( less than 0.5 xcexcm/coating) on dense/porous substrates for solid oxide fuel cell and gas separation applications. The method comprises first preparing a precipitate-free starting solution containing cations of oxide""s constituents dissolved in an aqueous mixture comprising a polymerizable organic solvent. The precursor film is deposited on the substrate by a spin-coating technique. The deposition is followed by drying and calcining in the presence of oxygen and at the temperatures below 600xc2x0 C. to convert the film of polymeric precursor into the metal oxide film.
All of the foregoing techniques have limited applicability to the fabrication of tubular ceramic membranes in that it is difficult to utilize such techniques to apply a dense layer on the inside of a tubular substrate. A major disadvantage of colloidal sol., slurry or polymeric precursor liquid based process is the difficulty of effectively coating substrates of large pore size, especially when the pore size exceeds 20 xcexcm. Usually, multiple coatings or different fabricating steps are needed to reduce the surface pore size and/or porosity to avoid the penetration of the coating solution. Additionally, these techniques generally require the close matching of shrinkage between the coating and substrate upon firing.
As will be discussed, the present invention provides a method useful in the manufacture of a composite structure that is very amenable to fabricating tubular composite structures having an internally located thin film layer. Furthermore, unlike prior art techniques, the present invention is capable of forming composite structures in which thin films are deposited on large pore size substrate. Other advantages will be apparent from the following discussion.
The present invention provides a cold isopressing method in which at least two layers of material are isopressed within an isopressing mold, thereby to laminate the at least two, layers and to compact at least the one of the at least two layers. One of the at least two layers consists of a tape-cast film. A further of the at least two layers can be formed by a granular form of a material.
The present invention can be applied to form ceramic membrane elements in which all layers contain ceramic materials (which can be different or the same materials) capable of oxygen or hydrogen selectivity at high temperatures. In such case, the resultant green ceramic would be removed from the isopressing mold and then fired to remove organic additives such as binders and plasticizing agents and then sintered to form a composite ceramic in accordance with known methods. Alternatively, the isopressing mold being fabricated from a plastic or rubber might be left intact and burned away during firing or sintering.
The invention is not, however, limited to the fabrication of ceramic membranes. The process of the present invention can be adapted to fabricate any composite film devices including non-oxygen conductor composite porous membranes for gas or liquid separation and filtration applications, e.g. carbon or zeolite composite films. Also, the materials of and the porous support and membrane can be other ceramic or metallic materials or mixtures thereof. Furthermore the application of present invention is not limited to any particular shapes. For instance, plates, rods, bars and tubes fall within the purview of the present invention.
Although the isopressing mold can be a known flat isopressing mold to form a sheet-like composite, it can also be of cylindrical configuration with the at least two layers being of coaxial cylindrical layers. The one of the at least two coaxial cylindrical layers can be formed by wrapping the tape-cast film around a mandrel of the isopressing mold. The tape-cast film can contain a ceramic. A further of the at least two coaxial cylindrical layers can be formed by introducing a granular material that can be a ceramic forming material into an annular space defined between the tape-cast film and an outer, cylindrical pressure bearing element of the isopressing mold. As used herein and in the claims, the term xe2x80x9cgranular formxe2x80x9d means either a powder or a powder mixed with additives such as in the case of a ceramic, plasticizing agents, binders, and etc. Further, the term xe2x80x9cceramic forming materialxe2x80x9d means either a ceramic powder or a ceramic powder mixed with additives. In case of an application of the present invention to the fabrication of ceramic membranes, the tape-cast film wrapped around the mandrel of the isopressing mold can form a dense layer inside the finished tubular, ceramic membrane to overcome the problem of forming such a layer on the inside of a tubular form. It should also be pointed out that since the tapes are in plastic form, there is no penetration of the solution into an adjacent layer. As a result, the adjacent support layer can be fabricated with large pore sizes, namely, above 20 xcexcm.
Another possibility contemplated by the present invention is that the other of the at least two coaxial cylindrical layers is a compacted tube of a granular form of a material, again possibly a ceramic forming material. In such case, the one of the at least two coaxial cylindrical layers is then formed by wrapping the tape-cast film around said compacted tube. In this regard, such a compacted tube might have sub-layers containing pore forming materials to provide active and inert supporting layers to a dense layer formed by the tape-cast film. As a result, a dense layer can also (or in addition to a dense layer on the inside of a tube) be formed on the outside of such a tubular membrane.
The tape-cast film can comprise a mixed conducting oxide capable of conducting one of hydrogen and oxygen ions and the other of the at least two coaxial cylindrical layers can contain a pore forming material. Thus, the present invention can be used to form a dense layer supported by a porous support. The other of the at least two coaxial cylindrical layers can also contain the mixed conducting oxide so that such support constitutes an active support. However, both layers do not have to have the same mixed conducting oxide and as such, the other of the at least two coaxial cylindrical layers can contain another mixed conducting oxide.
Preferably, the one of the at least two layers has a first thickness of between about 10 and about 200 xcexcm and the other of the at least two layers has a second thickness of between about 0.2 and about 5.0 mm after lamination.
The mixed conducting oxide can have a structure given by the formula: AxAxe2x80x2xxe2x80x2Axe2x80x3xxe2x80x3ByBxe2x80x2yxe2x80x2Byxe2x80x3O3xe2x88x92z, where A, Axe2x80x2, Axe2x80x3 are chosen from the groups 1, 2, 3 and the f-block lanthanides; and B, Bxe2x80x2, Bxe2x80x3 are chosen from the d-block transition metals according to the Periodic Table of the Elements adopted by the IUPAC. In the formula, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6xxe2x80x2xe2x89xa61, 0xe2x89xa6xxe2x80x3xe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6yxe2x80x2xe2x89xa61, 0xe2x89xa6yxe2x80x3xe2x89xa61 and z is a number which renders the compound charge neutral. Preferably, each of A, Axe2x80x2, and Axe2x80x3 is magnesium, calcium, strontium or barium.
Another possible structure for the mixed conducting oxide is one given by the formula: Axe2x80x2sAxe2x80x3tBuBxe2x80x2vBxe2x80x3wOx where A represents a lanthanide, Y, or mixture thereof, Axe2x80x30 represents an alkaline earth metal or mixture thereof; B represents Fe; Bxe2x80x2 represents Cr, Ti, or mixture thereof and Bxe2x80x3 represents Mn, Co, V, Ni, Cu or mixture thereof. Each of s, t, u, v, and w represent a number from 0 to about 1. Further, s/t is between about 0.01 and about 100, u is between about 0.01 and about 1, and x is a number that satisfies the valences of A, Axe2x80x2, B, Bxe2x80x2, and Bxe2x80x3 in the formula. Additionally, 0.9 less than (s+t)/u+v+w) less than 1.1.